Okay - after some testing I got it working. Smart switch and all.

Here's how it's wired.

It's not done 100% correct. The 3-way switch did not have a neutral. It seems the wiring went from the circuit box to the outside switch, with runners and a load line sent to the indoor switch (the one pictured). So to get around that I used the neutral wire for the inside single throw switch.

This is not 100% correct because those two switches are on two circuits. So I have added a note in my circuit box that if you're working on one of those circuits to flip the second and vice versa.

But they are connected and working!